The basic principles for making citrate solublecalcined alkali phosphates for use as fertilizers in a revolving furnace have been disclosed in German Pat. No. 481,177. According to this disclosure a mixture of crude phosphate, alkali carbonate and silicic acid is subjected to a calcining operation at a temperature of about 1200.degree.C. The amounts of starting components are selected to provide for at least 1 mol alkali oxide per mol P.sub.2 O.sub.5 and silicic acid is present in an amount to permit formation of calcium orthosilicate by combining with 1 mol of CaO derived from the tricalcium phosphate and with lime not attached to phosphoric acid in the starting mixture. The chemical mechanism presumably is as illustrated by the following reaction: EQU 2 Ca.sub.3 (PO.sub.4).sub.2 + SiO.sub.2 + 2Na.sub.2 Co.sub.3 = 2 (Na.sub.2 O .sup.. 2 CaO .sup.. P.sub.2 O.sub.5) + (2 CaO .sup.. SiO.sub.2) + 2 CO.sub.2
later investigations have shown that it is advantageous for a complete alkaline decomposition of natural calcium phosphates, briefly called herein "crude phosphates", if the mol ratio between the P.sub.2 O.sub.5 present in the crude phosphate and the alkali oxide used as decomposition agent is between 1 : 1.1 and 1 : 1.5. The reaction is favored by the presence of water vapor which can be supplied using fuels rich in hydrogen, particularly oil, in order to obtain the necessary decomposition temperature. The calcined product obtained in this manner at about 1200.degree.C is a slightly sintered, not molten porous mass. The products for this reason are known under the designation "sinter phosphates".
If calcined sodium carbonate is used a sodium-calcium silicophosphate is obtained of which the phosphoric acid content is virtually completely soluble in a 2% citric acid solution, in neutral ammonium citrate solution and particularly in an ammoniacal ammonium citrate solution which latter is also known as Petermann's solution. The fertilizer grade, that is the activity as fertilizer, of these calcined phosphates is best evaluated on the basis of the P.sub.2 O.sub.5 -solubility in Petermann's solution.
Although theoretically in general alkali carbonates can be used as decomposition agents, only sodium carbonate has found acceptance in industrial technique. As against postassium carbonate, sodium carbonate has the advantage that it is available at a lower price and can be handled more easily. Besides when sodium carbonate is used the calcining temperature can be increased to above 1200.degree.C without causing the melting to start. On the other hand when potassium carbonate is used the temperature should not exceed 1150.degree.C. Besides it is known that potassium compounds at the necessary reaction temperatures are largely volatile which causes rather high potassium losses during the comparatively long decomposition periods. Potassium containing fertilizers have therefore been made by simply adding potassium salts such as potassium chloride to the products obtained in the sodium carbonate decomposition.
There have also been repeated attempts to obtain calcined potassium phosphate directly in a calcining operation. In French Pat. No. 1,189,733 for instance it has been suggested to make calcined phosphates by calcination of crude phosphate, silicic acid and particularly potassium bicarbonate as decomposition agent at temperatures between 550.degree. and 900.degree.C. This, however, is only possible if the potassium bicarbonate is employed in a substantial excess compared with the usual decomposition with sodium carbonate. The thus obtained product has a comparatively high K.sub.2 O content. Because of the virtually complete water solubility of K.sub.2 O a strongly alkaline medium is produced when dissolved which in turn raises problems for the use as fertilizer.
In Belgian Pat. No. 605,561 an attempt has been made to make the process for producing calcined potassium phosphates more profitable by employing a starting mixture of crude phosphate and particularly potassium bicarbonate upon addition of water and granulating the mixture. However even when using the easier decomposable aluminum calcium phosphates as crude phosphates the results have not been satisfactory. In this patent it has also been proposed to employ solid potassium hydroxide as decomposition agent. This proposal however could not be carried out since such granulates cake together immediately upon introduction into the revolving furnace and stick to the furnace wall.
The German published application No. 1,925,539 has proposed the use of potassium hydroxide solutions, for instance, solutions obtained in the electrolysis of potassium chloride solutions. Accordingly, a granulate is formed from ground crude phosphate, potassium hydroxide solution and sand in conventional manner by mixing, granulating, drying and recirculating a suitable amount of the comminuted dried product. The decomposition by calcination is carried out at temperatures between 850.degree. and 1000.degree.C. It can easily be seen that this is not an industrially useful process because the preferred embodiment is to the effect that the calcining operation should be carried out on a moving sinter belt while adding ground coal. The sintering times according to the examples are 1 hour at 950.degree.C.
Although the application mentions that the calcining process can also be carried out in a revolving furnace it is quite apparent that such furnace cannot be used for technical reasons since the required reaction conditions, that is the maintaining of a calcining temperature of for instance 950.degree.C for a time in excess of 1 hour, can hardly be implemented in a revolving furnace. Therefore, when such furnace is used an incomplete decomposition must be expected. Besides, the critical point which makes the use of a revolving furnace inopportune is that, as indicated in the disclosure of the cited publication, the decomposition mixture will not withstand the mechanical processing in a revolving furnace. Tests by the present applicants have shown that this type of granulates will immediately cake together when introduced into the hot revolving furnace and will form heavy deposits on the furnace walls. The compacting of the product after a short time proceeds to the point where the furnace becomes clogged up and an interruption of the operation therefore becomes necessary.
In more recent times processes have also been proposed which permit use of concentrated aqueous alkali hydroxide solutions as decomposition agents in the making of calcined alkali phosphates in revolving furnaces. Basically these processes consist in converting the mixture of crude phosphate, sand and alkali hydroxide solution to a solid granulate or agglomerated product prior to introduction in the furnace. Such granulate or agglomerate will easier withstand the stress during passing through the furnace and, under the conditions in the revolving furnace will lead to products of high citrate-soluble P.sub.2 O.sub.5 content.
A number of proposals have been made to the effect to employ for this purpose the waste gases obtained in the revolving furnace operation which contain, in addition to carbon dioxide, an inert gas, volatile compounds formed during the calcining operation and dust particles. For this purpose the aqueous alkali hydroxide solutions are treated with the furnace gases prior or after mixture with the crude phosphate and the necessary amounts of silicic acid.
For instance according to the German Pat. No. 1,266,768 the potassium hydroxide solution is converted with the off gases from the furnace into an alkali carbonate solution of such concentration that a granulate adapted for the calcining process is formed after mixing with the crude phosphate and the required amounts of sand.
According to German published application No. 1,592,690 only a partial carbonization of the alkali hydroxide solution is effected in addition to the concentration. The formation of the granulate is effected subsequently by mixing the solution with the crude phosphate and the sand while passing therethrough the off-gases from the furnace.
From Belgian Pat. No. 713,005 it appears furthermore that suitable products from liquid and up to slurry-like mixtures of alkali hydroxides, crude phosphates and sand can be obtained by spray drying or drying on a belt by means of the hot off-gases from the furnace.
In British Pat. No. 1,159,650 it has been proposed to process the initial mixtures, preferably in the presence of off-gases from the furnace, with a sufficient amount of recirculated final product to obtain a granular starting material (also in U.S. Pat. No. 3,552,944).
According to German published application No. 1,294,977 a portion of the compound which furnishes the alkali oxide is introduced into the revolving furnace in the form of a concentrated aqueous alkali hydroxide solution by direct introduction near the point of introduction of the raw materials and the remainder is then added in the form of alkali carbonate. This operation is possible by bringing the alkali hydroxide solution into contact with a crude phosphate-alkali carbonate-sand mixture. The ratios in this case will depend on the type and concentration of the alkali hydroxide solution and the properties of the crude phosphate and alkali carbonate.
It is accordingly an object of the present invention to provide for a process of the type described wherein the entire amount of alkali hydroxide furnishing compound can be introduced directly into the revolving furnace in the form of an aqueous alkali hydroxide solution.